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       im_heq,  im_lhisteq,  im_lhisteq_raw,  im_hsp  - process an image using
       grey level transformations


       #include <vips/vips.h>

       int im_heq( in, out, bandno )
       IMAGE *in, *out;
       int bandno;

       int im_lhisteq( in, out, xw, yw )
       IMAGE *in, *out;
       int xw, yw;

       int im_hsp( in, ref, out )
       IMAGE *in, *ref, *out;


       im_heq(3) histogram equalises the unsigned char image held by the IMAGE
       descriptor  in.  The result is written to the IMAGE descriptor out.  If
       bandno is -1 then all input bands are equalised independently.  In  all
       other  cases the input image is equalised using the histogram of bandno
       only.  The latter processing produces better results.

       im_hsp(3) maps in to out with histogram  specified  by  the  ref.   All
       images  should  be  unsigned  char.   Each  band of the output image is
       specified according to the distribution of grey levels of the reference
       image according to im_histspec(3).

       im_lhisteq(3)  histogram  equalises the one channel unsigned char image
       pointed to by the Image descriptor in.  The result is  written  to  the
       IMAGE  descriptor out.  The histogram equalisation is based on a window
       of size xw by yw centered at the current location of each input  pixel.

       In  the  manner  of  im_conv(3) the input image is expanded so that the
       output image is the same size as the input.

       im_lhisteq_raw(3) is as above, but does not expand the input.


       All functions returns 0 on success and -1 on error.


       im_histgr(3),    im_histplot(3),    im_histspec(3),     im_lineprof(3),


       1991--1996 The National Gallery and Birkbeck College

                                  10 May 1991                        IM_HEQ(3)